The present invention generally relates to tests for the presence of chemical substances and, more particularly, to a novel test for the presence of sol-gel on aluminum.
Aluminum surfaces in aircraft, particularly on fasteners such as rivets, are typically coated at some stage during the manufacturing process with some kind of protective or adhesion promoting coating, called a conversion coating. Chromate conversion (such as ‘ALODINE® or IRIDITE®’) is a chemical treatment process for aluminum used to provide corrosion protection and surface preparation for paint and adhesives. There are a wide variety of conversion coatings, widely known in the industry generically as “alodine” coatings. Alodine coatings have been in wide use in the aircraft and aerospace industries since the 1950's. For example, various forms of alodine may include chromic acid or chromium trioxide or hexavalent chromium. There are a number of different formulations of alodine—such as ‘ALODINE 600, ALODINE 1000, ALODINE 1200S, ALODINE 1600 AND IRIDITE 14-2’ for example—that are commercially available from a large number of suppliers.
Conversion coatings are commonly used to protect rivet fasteners in the aerospace industry. Conversion coatings are typically used to protect the parts—for example, rivets—during storage prior to installation. Protection may be required, for example, to prevent corrosion of the aluminum surface of parts. Depending on the processes used, the conversion coating may also be used to provide an appropriate base on which to adhere paint.
“Rivet rash” refers to the loss of paint from aluminum rivet heads on in-service aircraft. Rivet rash is a serious problem that can greatly detract from an aircraft's appearance and airlines, especially, tend to be concerned over the detraction from decorative appearance in areas most visible to the passengers, in particular, the nose section and entry doors of aircraft. In addition, rivets can “rash” within six months of delivery giving the airplane a prematurely older appearance. The problem has been exacerbated by the introduction of newer, environmentally safer paints.
Because the presence or absence of conversion coatings is not always easily detectable, tests have been developed for them to verify whether or not the coatings are present on parts when, for example, there may be some reason for doubt or, for example, a part needs to be double-checked during a manufacturing process.
Due to problems with paint adhesion, aircraft manufacturers are developing new coatings and processes to improve paint adhesion on exterior aluminum surfaces, and especially for aluminum rivet heads. Some of the new coatings are clear and extremely thin, thus requiring special tests for the verification of their presence before parts are installed.
As can be seen, there is a need for a test to determine whether these new coatings are present or absent on aircraft parts. Moreover, there is a need for a quick, easy, and reliable way to verify that aluminum rivets have been coated with these new coatings after they arrive from the manufacturer.